It is more than Seam documentation related to JBoss Tools related.
You need JBoss Tools plugins installed in Eclipse or use bundled plugins in JBoss Developer Studio. Then after importing existing seam-gen generated project into IDE, you have to change/check the Seam Runtime which you want to use and which you have installed/configured in Eclipse JBoss Tools Seam plugin.
Use right mouse button click on your project and select Properties. Then go to seam Settings and Choose appropriate Seam runtimes from list or Add new one by clicking on Add button right next to listbox.
Then you have attached to your project correct build launcher - Seam Builder and you need also WTP facet, which you can easily add by configure JSF capabilities - Right mouse button click on project Configure - Add JSF Capabilities. Just choose correct Server Runtime (in your case JBoss AS 5.1) and that's it.